The present invention relates to an electrographic copying apparatus, more particularly, it relates to improvements of an exposure device of the copying apparatus.
Various types of electrographic copying apparatus are presently used. However, conventional electrographic copying apparatuses have the following disadvantageous points in the exposure device thereof.
Firstly, the temperature of an exposure lamp is excessively increased if the lamp is lit accidentally for a long time. Such a problem is governed by a law directed to the control of electric apparatus in Japan. Various means have been applied for avoiding the increase of the temperature of the exposure lamp lit for a long time. One of the means is to cool the lamp by a cooling mechanism. Another means is to adjust the periodic duty of the lamp. Still another means is to reduce the electric power of the lamp. However, such conventional means do not avoid accidental or unintended lighting of the lamp caused by improper handling of the apparatus due to the carelessness of an operator. Therefore, there is a possibility of thermal damage to the apparatus elements due to the carelessness of an operator.
Secondly, there is a problem in how to control the exposure and adjust the uneveness of the exposure in order to get a clear copy. In the exposure device of an electrographic copying apparatus in which an original is exposed and scanned by moving an optical system or moving the original, a beam from an exposure lamp is reflected by the original on an original positioning plate of glass. The beam reflected by the original exposes a photosensitive paper corresponding to the image density of the original through the optical system. It is desirable to control the exposure in response to the speed of the photosensitive paper, developing capacity, or material of the original. Also, it is desirable to control the uneveness of the exposure, due to the uneveness of the brightness of the exposure lamp and/or the characteristic of a lens that the brightness of the light beam through the lens is darker in the periphery portion of the lens than in the center, so as to expose the paper evenly, corresponding to the image density of the original.
A conventional means for controlling the exposure and the uneveness of the exposure comprises a movable guide plate and a cover plate attached to the end of the guide plate. The passage of the exposure beam is defined by the movable guide plate and a fixed guide plate facing the movable guide plate. The cover plate shields a part of the area of the cross section of the beam passage. The exposure, i.e. the amount of light used to expose a photosensitive paper, is controlled by shifting the movable guide plate so as to change the width of the beam passage. The uneveness of the exposure is controlled by cutting the cover plate to form a desirable shape, so that the shape of the cross section of the beam passage is changed. The movable guide plate is attached to a body of the exposure device by a screw positioned on the front side of the body and by another screw positioned on the rear side of the body. In such a conventional control means, the position of the movable guide plate is adjusted both at the front side and at the rear side. Such an adjusting operation is very troublesome. That is, it is not easy to equally shift the guide plate both at the front and rear sides. In addition, the cover plate must be removed from the guide plate and taken out of the exposure device, when the cover plate is to be cut for controlling the uneveness of the exposure. Such a control operation is, also, very troublesome. Furthermore, it is very difficult to take the cover plate out of the exposure device, if the space around the copying apparatus is small.
The third problem is how to prevent a shade from being copied along an edge of an overlapped paper on an original. In the conventional copying apparatus, the exposure device is movably mounted between an original positioning plate and a photosensitive paper. If an original on which another original paper overlaps is exposed, shade appears along the edge of the overlapped paper due to its thickness. This unnecessary shade is copied on the photosensitive paper. Especially, in an electrographic photoengraving machine, the shade line is copied on a master plate. Such a shade line must be erased from the master plate before an etching process of the master plate. This operation is very troublesome.